Dual conversion tuners have been implemented in providing tuning with respect to broadband signals for a number of years. Typically, the signal input to such a dual conversion tuner is a radio frequency (RF) signal which must be converted to a particular frequency, e.g., baseband frequency, for further signal processing. For example, it is not uncommon to utilize a dual conversion tuner, having a up-converter (mixer providing conversion of an RF input signal to a first intermediate frequency (IF)) and a down-converter (mixer providing conversion of the first IF to a second IF or a baseband signal), in a cable television set-top box to provide tuning with respect to a single cable television channel from a broadband cable signal including one hundred or more cable television channels.
In order to accommodate a broadband signal and provide a desired level of isolation with respect to a particular signal therein, various filters may be implemented with respect to a dual conversion tuner. For example, a common technique is to provide a first IF filter in the signal path between the up-converter and the down-converter. Other filters may additionally be used, such as a second IF filter coupled to the output of the down-converter to provide filtering of images or other spurious signals.
In typical prior art implementations, the aforementioned first IF filter has a precise (low tolerance) and fixed center frequency. Such a first IF filter configuration is utilized in order to provide an extremely high quality (Q) factor filter (providing sharp cutoff characteristics) having a very narrow bandwidth. Accordingly, the first IF filter may be utilized to filter unwanted channels of a broadband input signal in order that the down-converter portion of dual conversion receiver may provide linearity over a relatively small spectrum, e.g. three cable television channels, in contrast to the up-converter portion of the dual conversion receiver's ability to handle the full input spectrum.
The tolerance of the first IF filter in the aforementioned prior art configuration must typically be very low (the center frequency must be very close to a selected frequency) in order to avoid substantial attenuation of a desired signal associated with the sharp cutoffs of the pass band. For example, if the first IF is selected to be 1,250 MHz, a high tolerance first IF filter might provide a center frequency appreciably divergent from this selected center frequency (e.g., 1,100 MHz) and, therefore, a desired channel of the broadband input signal may be up-converted to 1,250 MHz only to be greatly attenuated (e.g., on the order of 40 to 50 dB) by the sharp cutoff characteristics of the first IF filter. Such a situation is very undesirable from system noise performance perspective.
In order to provide a filter having an extremely high Q factor and very narrow bandwidth with a precise center frequency, a surface acoustical wave (SAW) or ceramic resonator structure are often used. However, these filter implementations, by their very nature, are not provided on a same substrate as the aforementioned up-converter and/or down-converter, thereby making a fully integrated tuner solution impossible where such filters are used. Moreover, these filter implementations typically require significant expense in packaging the filters themselves as well as appreciable resources, e.g., circuit board space and power, in order to deploy them. SAW filters, for instance, typically require very tightly controlled, hermetically sealed packaging and are very temperature and pressure dependent. Accordingly, the packaging associated with such filters generally has an extremely high cost associated therewith.
Further adding to the cost of such filters are the costs associated with providing a precise center frequency. In addition to the costs involved in providing such a high quality filter, cost issues are presented by the yield factor associated with manufacturing such precise filters. For example, because the center frequency of the filter is required to be so accurate, a number of filters manufactured will be unusable because their center frequency falls outside the tolerance of the filter.
Implementing a discrete component filter to meet the above first IF filter performance characteristics, such as using a printed circuit (PC) board, also results in difficulty. For example, a PC board implementation, utilizing discrete inductors (such as may be printed upon a typical PC board) and/or capacitors (such as may be provided as manufactured packages), may be a relatively large, usually difficult to tune, solution. Discrete capacitors often introduce tolerances as large as 5 or 10%, depending on the technology used. The tolerance of such components carries through to the tolerance of the first IF filter, resulting in an implementation of the first IF filter having a relatively large tolerance. Similarly, inductors utilized in such a discrete component filter configuration present tolerance issues with respect to the filter. For example, inductors may be printed on a typical PC board, such as using a ¼ wavelength resonator. The ¼ wavelength resonators will resonate at particular frequencies which are directly related to the tolerance of the PC board manufacturing process. Although the tolerance of the PC board manufacturing process can be very tightly controlled at an expense, implementing ¼ wavelength resonators will nonetheless require a relatively large surface area for deployment. Accordingly, a small footprint tuner, such as may be required by some of today's electronic component requirements, may not be possible with such an implementation. Moreover, the tolerances of the individual components may result in a filter configuration which must be individually tuned in order to provide a desired center frequency.
Filters might be implemented in an integrated circuit, such as upon a same substrate as the aforementioned up-converter and down-converter; however an on-chip first IF filter produced on silicon presents issues very similar to those of the PC board implementation discussed above. Moreover, the components used to make the filter, such as on-chip capacitors and on-chip inductors, have very wide tolerances, typically appreciably more so than those of the PC board components discussed above. For example, capacitors implemented in silicon sometimes will present tolerances on the order of ±20%. However, such integrated circuit filter implementations typically are not tunable. Moreover, integrated circuits generally do, not provide sufficient available space in which to implement particular components, such as the aforementioned ¼ wavelength resonators, often resulting in Q factor issues with respect to the resulting filter.
It should be appreciated that in addition to the above mentioned difficulties associated with each particular filter implementation, the operating characteristics of such filters tends to drift over time. In the past, there has been no technique by which to detect a shift in the center frequency of such a filter and to compensate for such a change.